It is known that the dewatering of a fiber material web can be increased if heat is used in addition to mechanical pressing. Thus a pressing section is known from U.S. Pat. No. 4,163,688 in which a steam blower box is arranged in the vicinity of a suction roller. The web of fibrous material can be heated sufficiently with the aid of steam that the dewatering performance of the subsequent pressing zone is increased.
While exploiting this effect the fiber material webs are often pressed against a heated drying roll via a felt band. The water vapor which thereby arises at the roll moves away from the roll surface and tears away in the direction of the felt band the water which is present in the fibrous web. During this the problem however arises that the steam pressure, which depends on the contact pressure and on the roll temperature, on the one hand promotes dewatering but, on the other hand, can lead to delamination of the fiber material web. Delamination is the description for the breaking up of the fiber material web after leaving the press gap as a result of the sudden pressure relief.
The method described in German Offenlegungsschrift 37 05 241 or in the U.S. equivalent U.S. Pat. No. 5,071,513, and also the associated apparatus, aims at providing a solution to the above problem of delamination in that the dewatering is effected in an extended pressure zone in which the web can be acted on in sections in the direction of through movement with differentially high pressures and temperatures. This is achieved essentially by several hydrostatic pressing elements which are arranged in series in the direction of through movement and are independently controllable.
As explained in U.S. Pat. No. 5,071,513, the full content of which is incorporated in the present application by way of reference, the pressure and temperature conditions in the first section as viewed in the travel direction of the fibrous web, can be selected such that the hydraulic pressure prevailing in the first section is higher than the equilibrium vapor pressure of the water contained in the fibrous web under the prevailing temperature conditions. A first portion of water is, then, merely squeezed out of the fibrous web into the porous band. In the following second section of the extended pressing zone, the pressure and temperature conditions are selected or adjusted such that the hydraulic pressure is higher than ambient pressure but lower than the equilibrium vapor pressure of water under the temperature conditions prevailing in the second section. Under these conditions, water vapor or steam is formed where the counter-roll contacts the fibrous web and such water vapor or steam formation is sufficient to expel or displace substantially the remaining or second portion of liquid from the fibrous web. In this manner, the fibrous web is relieved from the hydraulic pressure in the second section of the extended pressing zone so that the fibrous web can expand to a certain extent and does not exit from the extended pressing zone in a undesirable over-compressed state. Furthermore, the pressure and temperature conditions in the second section of the extended pressing zone can be selected or adjusted such that also at least part of the water which adheres to the fibers of the fibrous web is also evaporated and displaced or transported into the porous band if the temperature in the fibrous web is sufficiently high and condensation of the water vapor or steam within the fibrous web can be avoided. This beneficial effect is further enhanced when the fibrous web exits from the extended pressing zone and the pressure is further reduced to ambient pressure.
During passage through the extended pressing zone, the counter-roll transfers or looses heat to the through-passing fibrous web and other components of the dewatering apparatus. Under certain conditions, the heat loss may assume such extent that the temperatures are insufficient for the desired evaporation in the second section of the extended pressing zone.
A similar solution to that set forth in DE OS 37 05 241 is proposed in the later published International Application WO 91/00389 in which two hydrostatic pockets are provided in series in the press zone in a shoe member. Although this reference also recognizes that it is important for the pressure profile in the press gap to be controlled so as to avoid delamination, the arrangement proposed is mechanically relatively complex.
In addition to the added complexity and expense for the pressing elements of U.S. Pat. No. 5,071,513, the fact that the pressure can only be changed sectionally is also to be regarded as a disadvantage.